Qualia (from the Latin, meaning "what sort" or "what kind"; Latin and English singular "quale", pronounced KWAHL-ay) are most simply defined as qualities or feelings, like redness, as considered independently of their effects on behavior.

Whether qualia actually exist is a hotly debated topic in contemporary philosophy of mind. The importance of qualia in contemporary philosophy of mind comes largely from the fact that they are often seen as being an obvious refutation of physicalism. Much of the debate over their existence, however, hinges on the debate over the precise definition of the term, as various philosophers emphasize or deny the existence of certain properties.

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There are a number of different definitions for qualia. The definitions in use have changed over time. One of the simpler, broader, definitions is "The 'what it's like' character of mental states. The way it feels to have mental states such as pain, seeing red, smelling a rose, etc'"[1].

C. I. Lewis, in his book Mind and the World Order published in 1929, was the first to use the term "qualia" in its generally agreed modern sense. (His original definition was that qualia are the "recognizable qualitative characters of the given.") Frank Jackson later defined qualia as: "...certain features of bodily sensations especially, but also of certain perceptual experiences which no amount of purely physical information can convey."

Under definitions like these, which are quite broad, there can be little doubt that qualia exist [2]. However, definitions this broad make it difficult to discuss the precise nature of qualia, and their interaction with the mind and the environment. Some philosophers have made attempts at more precise, and possibly narrower, definitions of qualia, describing things whose existence is more controversial.

Daniel Dennett identifies four properties which are commonly ascribed to qualia. According to these, qualia are:

ineffable; that is, they cannot be communicated, or apprehended by any other means than direct experience.

intrinsic; that is, they are non-relational properties, which do not change depending on the experience's relation to other things.

private; that is, all interpersonal comparisons of qualia are systematically impossible.

directly or immediately apprehensible in consciousness; that is, to experience a quale is to know one experiences a quale, and to know all there is to know about that quale.

If qualia of this sort exist, then a normally-sighted person who sees red would be unable to describe the experience of this perception in such a way that a listener who has never experienced color will be able to know everything there is to know about that experience. Though it is possible to make an analogy, such as "red looks hot", or to provide a description of the conditions under which the experience occurs, such as "it's the color you see when light of such-and-such wavelength is directed at you," supporters of this kind of qualia contend that such a description is incapable of providing a complete description of the experience.

Another way of defining qualia is as "raw feels". A raw feel is a perception in and of itself, considered entirely in isolation from any effect it might have on behavior and behavioral disposition. In contrast, a "cooked feel" is that perception seen as existing in terms of its effects.

One key consequence of the claim that such things as raw feels can be meaningfully discussed — that qualia exist — is that it leads to the logical possibility of two entities exhibiting identical behavior in all ways despite one of them entirely lacking qualia. While very few ever claim that such an entity, called a philosophical zombie, actually exists, the mere possibility is sufficient to refute physicalism. Those who dispute the existence of qualia therefore necessarily dispute the existence of philosophical zombies.

There is an ancient Sufi parable about coffee which nicely expresses the concept: "He who tastes, knows; he who tastes not, knows not."

Arguments for qualia generally come in the form of thought experiments which are designed to lead one to the conclusion that qualia exist. For example, the inverted spectrum thought experiment invites us to imagine that we wake up one morning, and find that for some unknown reason all the colors in the world have been reversed. Furthermore, we discover that no physical changes have occurred in our brains or bodies that would explain this phenomenon. Supporters of the existence of qualia argue that, since we can imagine this happening without contradiction, it follows that we are imagining a change in a property which determines the way things look to us, but which has no physical basis. The argument thus claims that if we find the inverted spectrum plausible, we must admit that qualia exist. However, some supporters of the existence of qualia do not argue that this change in qualia is actually possible, they simply invite us to consider the possibility; perhaps the nature of the qualia is uniquely defined by the physical state of the brain.

In Frank Jackson's "Epiphenomenal Qualia" (Jackson 1982), Jackson offers what he calls the "Knowledge Argument" for qualia. One example runs as follows:

Mary the colour scientist knows all the physical facts about colour, including every physical fact about the experience of colour in other people, from the behavior a particular colour is likely to elicit to the specific sequence of neurological firings that register that a colour has been seen. However, she has been confined from birth to a room that is black and white, and is only allowed to observe the outside world through a black and white monitor. When she is allowed to leave the room, it must be admitted that she learns something about the colour red the first time she sees it—specifically, she learns what it is like to see that colour.

This thought experiment has two purposes. First, it is intended to show that qualia exist. If we agree with the thought experiment, we believe that Mary gains something after she leaves the room—that she acquires knowledge of a particular thing that she did not possess before. That knowledge, Jackson argues, is knowledge of the quale that corresponds to the experience of seeing red, and it must thus be conceded that qualia are real properties, since there is a difference between a person who has access to a particular quale and one who does not.

The second purpose of this argument is to refute the physicalist account of the mind. Specifically, the Knowledge Argument is an attack on the physicalist claim about the completeness of physical truths. The challenge the Knowledge Argument poses to physicalism runs as follows:

Before her release, Mary was in possession of all the physical information about colour experiences of other people.

After her release, Mary learns something about the colour experiences of other people. Therefore,

Before her release, Mary was not in possession of all the information about other people’s colour experiences, even though she was in possession of all the physical information. Therefore,

There are truths about other people’s colour experience which are not physical. Therefore,

Physicalism is false.

Finally, Jackson argues that qualia are epiphenomenal: that is, that they are causally inefficacious with respect to the physical world. Jackson does not give a positive justification for this claim—rather, he seems to assert it simply because it defends qualia against the classic problem of dualism. Our natural assumption would be that qualia must be causally efficacious in the physical world, but some would ask how could we argue for their existence if they did not affect our brains. If qualia are to be non-physical properties (which they must be in order to constitute an argument against physicalism), some argue that it is almost impossible to imagine how they could have a causal effect on the physical world. By redefining qualia as epiphenomenal, Jackson attempts to protect them from the demand of playing a causal role.

In his paper "Quining Qualia" and his book Consciousness Explained, Daniel Dennett offers an argument against qualia which attempts to show that the above definition breaks down when we try to make a practical application of it. In a series of thought experiments, which he calls "intuition pumps", he brings qualia into the world of neurosurgery, clinical psychology, and psychological experimentation. His argument attempts to show that, once the concept of qualia is so imported, it turns out that we can either make no use of it in the situation in question, or that the questions posed by the introduction of qualia are unanswerable precisely because of the special properties defined for qualia.

In Dennett's updated version of the "inverted spectrum" thought experiment, "the neurosurgical prank", you again awake to find that your qualia have been inverted—grass appears blue, the sky appears green, etc. According to the classical account, you should be immediately aware that something has gone horribly wrong. Dennett argues, however, that it is impossible to know whether the diabolical neurosurgeons have indeed inverted your qualia (by tampering with your optic nerve, say), or have simply inverted your connection to memories of past qualia. Since both operations would produce the same result, you would have no means on your own to tell which operation has actually been conducted, and you are thus in the odd position of not knowing whether there has been a change in your "immediately apprehensible" qualia.

Dennett's argument revolves around the central objection that, for qualia to be taken seriously as a component of experience—for them to even make sense as a discrete concept—it must be possible to show that

a) it is possible to know that a change in qualia has occurred, as opposed to a change in something else;

or that

b) there is a difference between having a change in qualia and not having one.

Dennett attempts to show that we cannot satisfy (a) either through introspection or through observation, and that qualia's very definition undermines its chances of satisfying (b).

Dennett also has his own response to the "Mary the color scientist" thought experiment. He argues that Mary would not, in fact, learn something new if she stepped out of her black and white room to see the color red. Dennett asserts that if she already truly knew "everything about color", that knowledge would include a deep understanding of why and how human neurology causes us to sense the "quale" of color. Mary would therefore already know exactly what to expect of seeing red, before ever leaving the room. Dennett argues that the misleading aspect of the story is that Mary is supposed to not merely be knowledgeable about color but to actually know all the physical facts about it, which would be a knowledge so deep that it exceeds what we can imagine and twists our intuitions.

Mary might be considered to be like a feral child. Feral children have suffered extreme isolation during childhood. Technically when Mary leaves the room, she would not have the ability to see or know what the color red is. A brain has to learn and develop how to see colors. Patterns need to form in the V4 section of the visual cortex. These patterns are formed from exposure to wave lengths of light. This exposure is needed during the early stages of brain development. In Mary's case, the identifications and categorizations of color will only be in respect to representations of black and white.

There are lots of famous people that have argued for and against the importance of the idea of qualia. But is there a significant amount of scientific consensus on either side of this issue? The open survey system at canonizer.com is being developed by a growing grass roots group of people to rigorously measure scientific consensus.

There is a topic on theories of consciousness getting started. All experts and non experts are invited to quantitatively communicate to everyone what they currently think on this issue.

Out of the gate the scientific consensus is clearly in the pro qualia Consciousness is representational and real camp with such distinguished supporters as Steven Lehar, John Smythies and a growing number of others. The supporters of that camp believe no other theory of consciousness will ever be able to match the amount of scientific consensus this camp will be able to maintain going forward, and also that eventually there will be demonstrable scientific proof that will convert all others to this camp.